Ground detectors

Current VHE and Extremely High Energy (EHE) γ-ray astronomy is driven by ground based detectors. Instead of interacting with incident γ-rays like space de- tectors, these instruments measure the products generated by the collision of very high energy γ-rays striking the atmosphere. These interactions produce cascades of particles. Among them, those that are charged and travel with a velocity larger than the local speed of light emit Cherenkov radiation. These showers of parti- cles are called EAS and are explained with a deeper scope in Appendix A. The measurement of the EAS properties, with different techniques depending on the energy range, allow the reconstruction of the direction and the energy of the orig- inal particle. The major problem affecting this technique is the huge background of cosmic rays, as they generate similar showers as γ-rays. Understanding intrin- sic differences between EAS from different primary particles becomes crucial for background suppression.

Depending on the energy range of interest, current γ-ray detectors use different techniques:

• Atmospheric Cherenkov detectors: These instruments measure the Cherenkov light produced in EASs. There are two different detection approaches cur- rently in use: sampling detectors and imaging detectors.

Sampling detectors consist on a grid of counters measuring arrival time and density of Cherenkov light arriving from the front of EASs over a wide re- gion on the ground. Using the intensity and relative times recorded on the different detectors, the primary particle original direction and energy can be reconstructed. The main disadvantage of these detectors resides on the poor information acquired from the development of the EAS, therefore differen- tiating γ-rays from background becomes problematic. Nowadays imaging detectors outperform this technique, and their contribution to the field is not very significant.

Imaging detectors, instead of sampling the front of the EASs, create an image of the shower as classical optical telescopes. With these snapshots is possible to study the evolution of the EAS along the atmosphere, and parametrize dif-

ferent properties of the cascade. These parameters allow to differentiate be- tween γ-ray and cosmic ray induced EASs , greatly improving signal to noise ratio, and therefore sensitivity. The first Imaging Atmospheric Cherenkov Telescope (IACT) able to detect an astronomical source was Whipple [91], in operation since 1968, detected the Crab Nebula in 1988. Nowadays. VHE As- tronomy is driven by the current generation of IACT such as the MAGIC [92], the H.E.S.S. [93] and the VERITAS [94]. See section 3.1 for more details about IACTs.

• Water Cherenkov detectors: This technique also measures Cherenkov photon production, although in this case in a different medium with higher refractive index, increasing photon emission. Grids of opaque water tanks are spread over a wide area, measuring the Cherenkov emission from particles generated in EASs. As particles need to enter the tank before they are absorbed, these detectors need to be located at very high altitudes. This technique uses no optical focusing so the Field of View (FoV) of the detectors is significantly larger than IACTs, although at the same time they have a worse angular resolution, as direction reconstruction uses only timing and the intensity of the footprint. Detectors can be built as a single water pool, like MILAGRO experiment [95], or a grid of smaller individual water tanks, improving resolution, like the future HAWC [96] Observatory. HAWC which be analyzed with a deeper scope in section 2.4.1.

• Particle counter matrices: These detectors directly measure EASs in- duced particles using matrices of counters. These counters use classic particle detectors recording the arrival time and direction of the incoming particles, and deriving from them the energy and direction of the primary. As an ex- ample the Tibet-AS [97] experiment covers an area of 36.000 m2 with 697 individual scintillation counters. The Argo-YBJ [98] experiment uses a full coverage detector consisting of a single layer of resistive plate counters. These detectors are able to observe in the VHE range, although their sensitivities are not as competitive as current IACTs or water Cherenkov experiments.

photons in EASs. There is a fluorescence isotropic emission along EAS pro- portional to the energy of the impinging particle. In the atmosphere, charged particles generated by EASs excite nitrogen molecules, which eventually de- cay producing photons in the ultra-violet and visible range. Detectors are able to collect this component from very long distances collecting the fluo- rescense light with mirror systems directed towards Photo-Multiplier Tube (PMT) detectors. When several detectors observe the same shower in coin- cidence, they can measure the intensity and time of arrival of the radiation, reconstructing the development of the shower along the atmosphere, infer- ring direction and energy of the primary cosmic ray. Taking into account the low efficiency of fluorescence emission in the atmosphere, only cosmic rays in the Ultra High Energy (UHE) and EHE range are observed with this tech- nique. This technique has been successfully applied in the High Resolution Fly’s Eye (HiRes) experiment [99], studying the composition of UHE cosmic rays.

• Hybrid detectors: These detectors take benefit from several of the pre- viously listed techniques at the same time. Each technique can be used to study different aspects of an EAS which then are combined. The Piere Auger Observatory [100], combines 1600 water Cherenkov detectors with 27 optical telescopes to measure fluorescence from UHE cosmic rays, observing energies beyond 1018_{eV over an area of 3.000Km}2_.